A control method for a variable volume pump is known in which, instead of an ordinary bleed control for controlling a hydraulic actuator speed by changing a bleed flow rate according to an operation amount of a control valve, a directional control valve of a closed center type is used, while a virtual bleed opening is set in the control valve and an area of the bleed opening (virtual bleed opening area) is changed according to the operation amount. According to the control method, a necessary pump discharge pressure is calculated using the virtual bleed opening area and a virtual bleed amount derived therefrom to perform the pump control such that the pump discharge pressure is implemented.
However, because only the virtual bleed opening is set and a negative control restriction is not assumed, a virtual negative control system is not replicated. As is generally known, the negative control system is in touch with human sensibilities, because the speed of the hydraulic actuator is low when a load is high while the speed of the hydraulic actuator is high when the load is low. On the other hand, if the virtual negative control system is replicated using a directional control valve of a closed center type, it becomes necessary to provide an unloading valve upstream from the directional control valve so as to discharge an excess flow rate from the hydraulic pump to the tank when the flow path in the directional control valve to the hydraulic actuator is closed. However, during discharging the excess flow rate with the unloading valve, the discharge pressure of the hydraulic pump becomes close to 0 because of little restriction. In this case, if the virtual negative control system is replicated based on such a discharge pressure of the hydraulic pump, such an command value (that instructs a maximum flow rate, for example) that causes the discharge flow rate of the hydraulic pump to increase is generated, which leads to a problem that energy is wasted.